The present invention relates to a hologram-recorded medium and a process for the fabrication of the same, and more particularly to a process for the fabrication of a computer-generated hologram in which interference fringes are formed on a given recording surface by computer-aided computation and a hologram-recorded medium obtained by the same.
In recent years, coherent light has been easily obtainable by use of lasers, and holograms have been widely commercialized as well. Especially for notes and credit cards, the formation of holograms on portions of their media has become popular for anti-counterfeiting purposes.
Today""s commercially available holograms are obtained by recording original images on media in form of interference fringes, using optical techniques. That is, an object that forms an original image is first provided. Then, light from this object and reference light are guided through an optical system such as a lens onto a recording surface with a photosensitive agent coated thereon to form interference fringes on the recording surface. Although this optical technique requires an optical system of some considerable precision for the purpose of obtaining sharp reconstructed images, it is the most straightforward method for obtaining holograms and so becomes most widespread in industry.
On the other hand, techniques for forming interference fringes on a recording surface by computer-aided computations for the fabrication of holograms, too, have been known to those skilled in the art. A hologram fabricated by such techniques is generally called a computer-generated hologram (CGH for short) or referred to simply as a computer hologram. This computer hologram is obtained by computer simulation of a process of generation of optical interference fringes, which process is all performed through computer-aided computations. Once image data on an interference fringe pattern have been obtained by such computations, physical interference fringes are formed on an actual medium. A specific technique has already been put to practical use, in which image data on a computer-generated interference fringe pattern are given to an electron beam lithographic system, so that the data are scanned by electron beams on a medium thereby forming physical interference fringes on the medium.
While keeping pace with recent developments of computer graphics, computer-aided processing of various images is being generalized in the printing industry. For the original images to be recorded in holograms, too, it is thus convenient to provide them in the form of image data. In consideration of such demands, techniques for generating computer holograms are of growing importance, and expected to take over optical hologram fabrication methods at some future time.
As explained above, significantly important commercial exploitations of holograms are to use them as anti-counterfeiting means for notes, credit cards or the like. To further enhance the anti-counterfeiting effect in such applications, it is effective to record a plurality of original images in the form of holograms. For instance, if the first original image comprising a pattern of size large enough for visual perception and the second original image comprising a pattern of visually unperceivable size are recorded on the same recording medium, then authentication can be carried out by observing the first original image. For the purpose of stricter authentication, it is possible to observe the second original image under loupes, microscopes or the like, thereby making more precise authentication. If micro-characters having a maximum size of up to 300 xcexcm are used for the second original image, they visually looks just like a simple striped pattern, but they can be perceived as characters under loupes, microscopes or the like.
In commercial applications, holograms are used as ornamental materials for commodities, for instance, in the form of cards, key holders, and ornamental articles. In such applications, too, it is effective to record a plurality of original images thereby improving the decorative effects of the holograms. For instance, if the first original image that is a master motive and the second original image functioning as a background pattern are recorded on the same recording medium, it is then possible to obtain a great-looking 3D appearance at the time of viewing. It is understood that three or more original images may be recorded as holograms in the same recording medium.
However, illumination environments for reconstruction of commercially exploited hologram-recorded media are usually far from ideal. By definition, the illumination environment ideal for reconstruction is an environment wherein a hologram is irradiated with illumination light comprising the same monochromatic light as used for recording reference light from the same direction as applied for recording. In the real world, however, hologram images are hardly reconstructed in such an ideal illumination environment. That is, in daily life, hologram images are reconstructed in illumination environments having a broad range of wavelengths, e.g., under sunbeams in outdoor conditions and under electric bulbs within rooms, and so reconstructed images are much inferior in sharpness to ideal reconstructed images. For this reason, when images are reconstructed from a hologram-recorded medium with a plurality of original images recorded therein, plural reconstructed images are prima facie obtainable, but individual reconstructed images lack sharpness, providing generally blurred, flat images. Especially with a hologram fabricated in such a way that a plurality of original images having master-slave relations are recorded with some intents, e.g., a hologram having a combined master motive and background pattern, they are observed in a fused state where the master-slave relations are little identified.
It is thus the primary object of the present invention to provide a computer-generated hologram that enables a plurality of original images to be viewed with master-slave relations as intended even when a hologram image is reconstructed in daily illumination environments, and a process for the fabrication of the same.
(1) According to the first embodiment of the present invention, there is provided a process of the fabrication of a computer-generated hologram with interference fringes recorded on a given recording surface by computer-aided computations, which comprises steps of:
defining 2 to K original images, a recording surface for recording the original images and reference light with which the recording surface is irradiated and which corresponds to the 2 to K original images,
defining a multiplicity of sample light sources on each original image,
defining a given angle of spreading for object light emitted from individual sample light sources,
determining an area on the recording surface, at which object light emitted from all sample light sources defined on one original image arrive with a limited angle of spreading, as a recording area corresponding to said one original image, thereby defining recording areas corresponding to each of the K original images,
assigning priorities to a plurality of recording areas when the plurality of recording areas overlap one another on the recording surface, so that a recording area having higher priority is preceded over the rest with respect to an overlapping portion, thereby eliminating the overlapping portion,
defining a multiplicity of computation points on the recording surface so that on each computation point, the intensity of interference fringes formed by reference light and object light emitted from sample light sources on the original image corresponding to the recording area to which said computation point is allocated and from which the overlapping has been eliminated is found by computation, and
forming interference fringes comprising a distribution of intensities of interference fringes found on each computation point as a hologram on the recording surface.
(2) According to the second embodiment of the present invention, there is provided a process of the fabrication of a computer-generated hologram with an optical pattern recorded on a given recording surface by computer-aided computations, which comprises steps of:
defining 2 to K original images and a recording surface for recording the original images,
defining a multiplicity of sample light sources on each original image,
defining a given angle of spreading for object light emitted from individual sample light sources,
determining an area on the recording surface, at which object light emitted from all sample light sources defined on one original image arrive with a limited angle of spreading, as a recording area corresponding to said one original image, thereby defining recording areas corresponding to each of the K original images,
assigning priorities to a plurality of recording areas when the plurality of recording areas overlap one another on the recording surface, so that a recording area having higher priority is preceded over the rest with respect to an overlapping portion, thereby eliminating the overlapping portion, and
defining a multiplicity of computation points on the recording surface, so that on each computation point, the complex amplitude at the position of said computation point of object light emitted from sample light sources on the original image corresponding to the recording area, to which said computation point is allocated and from which an overlapping portion has been eliminated, is computed thereby defining a specific amplitude and a specific phase on individual computation points, and locating a physical cell having optical properties consistent with the specific amplitude and specific phase in the vicinity of individual computation points so that a hologram-recording surface for the K original images is formed by a set of physical cells.
(3) According to the third embodiment of the present invention, there is provided a computer-generated hologram fabrication process according to the first or second embodiment, wherein:
the position of viewing a hologram reconstructed image is predetermined so that higher priority is assigned to a recording area corresponding to an original image located at a position nearer to the position of viewing.
(4) According to the fourth embodiment of the present invention, there is provided a computer-generated hologram fabrication process according to the first or second embodiment, wherein:
the position of viewing a hologram reconstructed image is predetermined so that higher priority is assigned to a recording area corresponding to an original image located at a position farther off the position of viewing.
(5) According to the fifth embodiment of the present invention, there is provided a computer-generated hologram fabrication process according to any one of the 1st to 4th embodiments, wherein:
the recording surface is located on an XY plane to limit the angles of spreading, xcex8x and xcex8y, of object light in the X-axis and Y-axis directions, said object light being emitted from each sample light source defined as a point light source toward the Z-axis direction.
(6) According to the sixth embodiment of the present invention, there is provided a computer-generated hologram fabrication process according to any one of the 1st to 4th embodiments, wherein:
a unit area having given size is defined so that the angle of spreading is defined for individual sample light sources in such a way that object light emitted from one sample light source reaches only within the unit area on the recording surface.
(7) According to the seventh embodiment of the present invention, there is provided a computer-generated hologram fabrication process according to any one of the 1st to 4th embodiments, wherein:
the recording surface is located on an XY plane, so that the angle of spreading in the X-axis direction of object light emitted from each sample light source defined as a point light source toward the Z-axis direction is defined as a given angle xcex8x and the angle of spreading of the object light in the Y-axis direction is defined as an angle satisfying a condition under which object light emitted from one sample light source reaches only within an area on the recording surface and having a given width Ly in the Y-axis direction.
(8) According to the eighth embodiment of the present invention, there is provided a computer-generated hologram fabrication process according to any one of the 1st to 4th embodiments, wherein:
the recording surface is located on an XY plane and a plurality of sections parallel with an XZ plane are located at a given spacing D, thereby defining sample light sources lined up at a given spacing on a sectional line obtained by cutting the surface of an original image by each section, and
each line of intersection of the recording surface with each section is allowed to have a given width thereby forming a strip area and the angle of spreading in the Y-axis direction of object light emitted from sample light sources lined up on a sectional line by a j-th section is defined in such a way as to satisfy a condition under which the object light reaches only within a strip area formed with respect to a line of intersection of the j-th section with the recording surface.
(9) According to the ninth embodiment of the present invention, there is provided a hologram-recorded medium in which a hologram optical pattern fabricated by the computer-generated hologram fabrication process according to any one of the 1st to 8th embodiments is recorded on a hologram medium such as a master medium written by an electron beam lithographic system, a replica made using the master medium, and a medium obtained by forming a reflecting layer on the replica.
(10) According to the tenth embodiment of the present invention, there is provided a hologram-recorded medium in which there is recorded information about a plurality of original images that are located at positions where projected images overlap upon projection onto a recording surface in a direction vertical thereto, wherein:
on the recording surface of the medium there are formed a plurality of recording areas that do not spatially overlap one another, and in one recording area there is recorded only information about object light emitted from a multiplicity of sample light sources that form one original image to be recorded.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts, which will be exemplified in the construction hereinafter set forth, and the of the invention will be indicated in the claims.